Paired Box Transcription Factors
The paired box (Pax) family of transcription factors all share a characteristic 128 amino acid DNA-binding paired domain. All Pax transcription factors also have a carboxy-terminal transactivation domain. The Pax gene family is split into subgroups based on the presence or absence of the following structural regions: a full homeodomain (60 aa helix-turn-helix), a partial homeodomain (first helix only), and/or an octapeptide motif. Pax3 and Pax7 contain all of the above structural regions. Pax4 and Pax6 have a full homeodomain but no octapeptide motif. Pax2, Pax5, and Pax8 have a partial homeodomain and an octapeptide motif. Finally, Pax1 and Pax9 have an octapeptide motif but no homeodomain. Pax gene-encoded transcription factors regulate tissue development and cellular differentiation in embryos and, in general, Pax expression ends when development is complete. Pax expression does persist in a few tissues in adult organisms to direct organ-specific regenerative events and protect against stress-induced cell death.